From MK.

"On the "most powerful" being:

Talos.

The HoonDing.

Trinimac.

Vivec.

Leki.

Reman.

Auri-El.

Wulfharth.

Morihaus.

Pelinal.

That's my list, and pretty much in that order. Though Vivec did kill Tiber Septim once...but I mentioned Talos, not the Emperor."
http://www.imperial-library.info/content/forum-archives-michael-kirkbride

I wish I could find the context of that post because I'm pretty certain it was elucidating.

Anyways, I wouldn't take it entirely seriously.
And I'm pretty sure the mention of Vivec killing Tiber Septim was just one of MK's numerous instances of dropping tantalizing hints at juicy stories we haven't heard yet. Although it could be a mention to some metaphysical event that I've failed to recognize.